1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography, such as an electrophotographic copying machine, an electrophotographic printer (e.g., a laser beam printer or an LED printer), a facsimile apparatus, and a word processor.
2. Description of the Related Art
As an image forming apparatus using electrophotography, an inline-type image forming apparatus using an intermediate transfer medium is known which forms a full-color image by a plurality of color toner images. In the inline-type image forming apparatus, for example, image forming stations 10Y, 10M, 10C, and 10Bk corresponding to a plurality of colors are respectively constituted by developing means, electrophotographic photosensitive drums 1Y, 1M, 1C, and 1Bk serving as first image bearing members, and process means that act on the drums, as shown in FIG. 9. The image forming stations 10Y, 10M, 10C, and 10Bk are arranged in a line so as to oppose an intermediate transfer medium 7 serving as a second image bearing member. Toner images of different colors are transferred one on another onto the intermediate transfer medium 7, and are transferred together onto a transfer material 13 by a secondary transfer means 8. This method is widely used because good output can be obtained, regardless of the type of the transfer material, and speedy formation of color images is possible.
When a monocolor image is formed in this image forming apparatus, the photosensitive drums 1Y, 1M, and 1C in the color-image forming stations 10Y, 10M, and 10C can be separated from the intermediate transfer medium 7 without rotation of the drums, as shown in FIG. 10. In this case, the use of the photosensitive drums 1Y, 1M, and 1C is avoided during formation of a monocolor image.
Japanese Patent Laid-Open No. 2004-4398 proposes a separation means that separates photosensitive drums Y, M, C, and Bk from an intermediate transfer belt in order to reduce the use of the photosensitive drums. Separation is performed after primary transfer of all toner images to be transferred onto the last sheet in one print job is completed, before the toner images are subjected to secondary transfer, and after secondary transfer onto the last second sheet is completed.
In the related art, however, a toner image must not lie at the second transfer position during separation, and therefore, there is a need to prohibit formation of a toner image for a period longer than the time for which the separating operation is completed.
It is common to connect a printer or a copying machine having a printer function to a network, and as a result, a plurality of users sometimes simultaneously make various print requests. For this reason, it is necessary to print a full-color image during a monocolor print mode, or conversely, to print a monocolor image during the full-color print mode.
In this case, switching between the modes must be performed so that image defects, such as color misregistration, are not caused by the influence of the operation of moving the intermediate transfer medium and the color-image forming stations into contact with or apart from each other.
That is, when the full-color mode is switched to a monocolor mode, the intermediate transfer medium and the color-image forming stations must be separated while a full-color image formed on the intermediate transfer medium does not lie at a primary transfer position in the black-image forming station and at a secondary transfer position. Similarly, when a monocolor mode is switched to the full-color mode, the intermediate transfer medium and the color-image forming stations must be brought into contact with each other while a monocolor image does not lie at the primary transfer position in the black-image forming station and at the secondary transfer position.
However, in normal continuous image formation, the non-image forming region on which a toner image is not formed (a region between image forming regions on which toner images are formed) is normally made small in order to maximize the number of prints to be continuously made. In most cases, the period in which image formation is prohibited is shorter than the contact or separation time of the intermediate transfer medium. For this reason, it is impossible that an image does not lie at both the primary transfer position in the black-image forming station and the secondary transfer position during the contact or separation time.
Accordingly, as shown in FIGS. 11 and 12, when the full-color mode is switched to a monocolor mode, the intermediate transfer medium 7 is separated from (taken out of contact with) the color-image forming stations 10Y, 10M, and 10C after a full-color image formed on the intermediate transfer medium 7 (a monocolor image when a monocolor mode is switched to the full-color mode) passes through the secondary transfer position, and image formation in a monocolor mode is then started. Therefore, when the mode is frequently changed, the number of output images produced per unit time is reduced, and output performance is seriously reduced.